```ts
/**
 * Merges overlapping intervals into the smallest list of non-overlapping intervals.
 * @param intervals - A list of [start, end] pairs.
 * @returns The merged intervals, sorted ascending by start.
 */
export function mergeIntervals(intervals: number[][]): number[][] {
  if (intervals.length === 0) return [];

  const sortedIntervals = intervals
    .slice()
    .sort((a, b) => a[0] - b[0]);

  const merged: number[][] = [];
  let currentInterval = sortedIntervals[0];

  for (let i = 1; i < sortedIntervals.length; i++) {
    const nextInterval = sortedIntervals[i];
    if (currentInterval[1] >= nextInterval[0]) {
      currentInterval[1] = Math.max(currentInterval[1], nextInterval[1]);
    } else {
      merged.push(currentInterval);
      currentInterval = nextInterval;
    }
  }

  merged.push(currentInterval);

  return merged;
}
```